Basic Components of Nucleic Acids – Purines and Pyrimidines

One of the important specialized pathways of amino acids and the synthesis of purine and pyrimidine nucleotides. These nucleotides are important for a number of reasons. Most of them, not just ATP, are the sources of energy that drive most of our reactions. ATP is the most commonly used source but GTP is used in protein synthesis as well as a few other reactions. UTP is the source of energy for activating glucose and galactose. CTP is an energy source in lipid metabolism. AMP is part of the structure of some of the coenzymes like NAD and Coenzyme A. And, of course, the nucleotides are part of nucleic acids. Neither the bases nor the nucleotides are required dietary components.

a) Nitrogenous Bases: The nitrogenous bases are the derivatives of two parent compounds. They are PURINES & PYRIMIDINES

Purines: Purine bases are heterocyclic compounds consisting of a pyrimidines ring and an imidazole ring fused together. The two purine bases are-

Adenine (6-Amino Purine): (C5H5N5), found in both RNA and DNA, is a white crystalline purine base, with Molecular weight 135.15 daltons and melting point 360 to 365 C.

Guanine (2-Amino-6-oxyPurine): (C5H5ON5), also found in both DNA and RNA, is a colorless, insoluble crystalline substance, with MW=151.15 dlatons. It was first isolated from guano (bird manure), hence so named.

Pyrimidines: Pyrimidine bases consist of six membered ring with two nitrogen atoms. The pyrimidine bases are –

Cytosine (2-Oxy-4-amino pyrimidine): (C5H6O2N5), found in both RNA and DNA, is a white crystalline substance, with MW=111.12 daltons and a melting point 320 to 325 C.

Thymine (2, 4-dioxy-5-methyl pyrimidine): (C5H6O2N2), found in DNA molecules only, has MW=126.13 Daltons. It was first isolated from thymus, hence so named. Only rarely does thymine occur in RNA.

Uracil (2, 4-dioxy pyrimidine) (C4H4O2N2), found in RNA molecules only, is a white, crystalline pyrimidine base with MW=112.10daltons and a melting point 338 C. Only rarely does uracil occur in DNA.

b. Sugar moiety:
Pentose sugar is present in DNA & RNA. It is present in their “β-furanose” from (close five number ring) and of β-configuration. Two types of pentose sugars present in the nucleic acid.

Ribose (RNA)

2-Deoxyribose (DNA)

c. Phosphate:
It contains the monovalent hydroxyl groups and one divalent oxygen atom all are linked to pentavalent phosphorous atom. The base is joined covalently (at N1 of pyrimidines and N9 of purines) and the phosphate is esterified to the 5’-carbon. The N-glycosyl bond is formed by removal of the elements of water (a Hydroxyl groups from pentose and Hydrogen atom from the base).